Unix Commands & Syscalls

$ ls -la myfile.c
-rwxr-xr-- 1 alice devs 1234 Jun 10 file.c
 ^  ^  ^  ^
 |  |  |  └── others: r-- = 4 (read only)
 |  |  └───── group:  r-x = 5 (read + execute)
 |  └──────── owner:  rwx = 7 (read + write + execute)
 └─────────── file type: - = regular, d = directory, l = symlink

# Octal notation: r=4, w=2, x=1
# rwx = 4+2+1 = 7,  r-x = 4+0+1 = 5,  r-- = 4+0+0 = 4
# So -rwxr-xr-- = 754 in octal

# chmod — change permissions
chmod 755 script.sh     # owner=rwx, group=r-x, others=r-x
chmod 644 data.txt      # owner=rw-, group=r--, others=r--
chmod +x  program       # add execute for all
chmod u-w readonly.c    # remove write from owner (u=user)

# chown — change owner
chown alice:devs file.c  # set owner=alice, group=devs

# Navigation
pwd                      # Print Working Directory — where am I?
cd /home/alice           # Change Directory (absolute path)
cd ..                    # Go up one directory
cd ~                     # Go to home directory
ls -la                   # List All files, Long format (permissions, size)

# File operations
mkdir -p dir/sub/leaf    # Make Directory, -p creates parents too
rm -rf dir               # Remove Forcibly + Recursively (dangerous!)
cp -r src/ dest/         # Copy Recursively
mv old.c new.c           # Move (also used to rename)
cat file.txt             # conCATenate — print file to stdout
head -n 20 file.txt      # First 20 lines
tail -n 20 file.txt      # Last 20 lines
wc -l file.txt           # Word Count, -l = line count only

# Searching
grep -r "main" src/      # Search recursively for "main" pattern
grep -n "TODO" *.c       # Show line numbers, all .c files

# Sorting and processing
sort names.txt           # Sort lines alphabetically
sort -n numbers.txt      # Sort numerically

# Man pages — ALWAYS check these
man ls                   # Manual for ls (shell commands)
man 2 open               # Manual section 2 = system calls
man 3 printf             # Manual section 3 = C library functions

/* Headers needed for all low-level I/O */
#include <fcntl.h>   // open(), O_RDONLY, O_WRONLY, O_CREAT flags
#include <unistd.h>  // read(), write(), close(), STDIN_FILENO etc.
#include <errno.h>   // errno variable + error codes (EINTR, ENOENT...)

/* open — opens a file, returns a file descriptor (int) */
int open(const char *pathname, int flags);
         ^               ^
         file path       O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_TRUNC
 returns: fd >= 3 on success, -1 on error (errno is set)

/* read — reads bytes from fd into buffer */
ssize_t read(int fd, void *buf, size_t count);
             ^       ^           ^
             fd      buffer      max bytes to read
 returns: bytes read (0 = EOF), -1 on error

/* write — writes bytes from buffer to fd */
ssize_t write(int fd, const void *buf, size_t count);
 returns: bytes written, -1 on error

/* close — releases the file descriptor */
int close(int fd);
 returns: 0 on success, -1 on error

/* Standard file descriptors — always open */
0  = STDIN_FILENO   // standard input (keyboard)
1  = STDOUT_FILENO  // standard output (terminal)
2  = STDERR_FILENO  // standard error (terminal)

#include <errno.h>
#include <string.h>  // for strerror()

ssize_t n = read(fd, buf, 100);
if (n < 0) {
    int err = errno;              // save immediately — next call may change it
    if (err == EINTR) {
        /* interrupted by a signal — safe to retry */
    } else if (err == ENOENT) {
        fprintf(stderr, "File not found: %s\n", strerror(err));
    }
}

/* Common errno values */
// ENOENT = No such file or directory
// EACCES = Permission denied
// EINTR  = Interrupted by signal (retry is safe)
// EBADF  = Bad file descriptor (fd not open)
// EAGAIN = Try again (non-blocking I/O not ready)

# strace runs a program and prints every system call it makes
strace ./myprogram
strace -e trace=read,write ./myprogram   # filter to specific calls
strace -p 1234                           # attach to running process PID 1234

# Example output of strace on "cat file.txt":
open("file.txt", O_RDONLY)    = 3
read(3, "hello\n", 4096)      = 6
write(1, "hello\n", 6)        = 6
read(3, "", 4096)             = 0  <-- EOF
close(3)                      = 0

# /proc/$PID/fd — see open file descriptors of a process
ls -la /proc/$$/fd   # $$ = current shell's PID
cat /proc/$$/status  # memory usage, state, signals